Fuel sending unit and Fuel Gauge

[Socnarftrucks]

1980 f100 flareside. Installed a brand new tank, hoses, and  brand new sending unit. My fuel gauge goes to full when I fill it, but it appears to eat a lot of gas (no more than 10 mpg) drops pretty  fast and when it's on E, it's actually only half empty. HELP!

[FuzzFace2]

Socnarftrucks wrote

1980 f100 flareside. Installed a brand new tank, hoses, and  brand new sending unit. My fuel gauge goes to full when I fill it, but it appears to eat a lot of gas (no more than 10 mpg) drops pretty  fast and when it's on E, it's actually only half empty. HELP!

I cant help in the MPG as my 81 also seams to goes through gas pretty fast.
I am happy I added a rear 19 gallon tank to the flare sides 16 gallon side tank.

Now on that sender were did you get it from?
Did you match it up to the old one?
I ask this because the short bed 16 gallon tank and the 19 gallon long bed tank use different senders and most places only list the long (wrong for short bed) bed sender for both long & short beds.

I did replace mine as part of my rebuild and got to say it reads right on the money.
5 gallons will get the needle just up to the empty line for both tanks, and full when filled up.

BTW I am only getting mid 14's with my 4x2 truck, 300 six, NP435 stick with advance adaptors over drive and 2.75 rear gear. I have a air fuel ratio gauge to install so I can tweak the carb for better MPG and mess with the timing too.
Dave ----

[Socnarftrucks]

Thanks. First time I got it from Amazon but it didn’t work at all. I called the manufacturers technical support and they told me pretty much what you said and sent me the correct replacement (right shape  and voltage they said). It wasn’t until I installed the bed and went for my first fill up where I noticed the discrepancy. I’d hate to drop the bed again. As far as the MPG i had expected arounfm 10-12. I have 4.9 with a C6

[FuzzFace2]

I did not read that you matched up the "right one" to the old one to make sure it was the same shape, not that the old one was right either?

The senders should only go in 1 way and I think there is a notch the sender has to fit so the float and pick up sit right inside the tank.

Before you added any gas to the tank did the needle sit on empty?
How much did you add and where did the needle sit then?
I see it being 1 of 2 things:
Float needs adjusting to read empty, I don't care if it does not read full all the way.
The sender was installed wrong and the float & pick up are not reaching the bottom of the tank.
I don't think the sender can be installed this way because the pick up hits the tank so???

Also before I put the bed back on I added 5 gallons to each tank just to see if the gauges would move so I would know just how low I could go before I ran out of gas.
Key off the needles are just below the empty line and with 5 gallons it sits on the line so I know I can go that low. I may do that on 1 tank but I don't do it when on the 2nd for fear of running out.

Sorry  to say I think if you want it fixed the bed or tank needs to be moved.
Dave ----

ps so you don't feel so bad just after I got my truck on the road the rear tank (new) sender stopped working.
In testing the sender looked to be the issue and when I dropped the tank (not fun even empty) and removed the sender I found the float filled with gas. I used the old sender float and it been working ever since.
I also now have a rear tank filler hose leaking on fill ups.

[Socnarftrucks]

It was marched. I probably need to adjust the floater. Looks like I’m dropping the bed again. Thanks

[Rembrant]

I spent quite a bit of time getting my sending unit to read correctly, or semi-correctly. I had to bend my float down quite a bit, so much so I also had to bend the pickup tube down further as well. It works now, not perfect but close enough.

[ratdude747]

Mine does the same thing to a lesser degree.  Full is full, empty is empty, but it races down fast but slows down as it approaches empty. Almost seems like the replacement sender has a logarithmic or exponential response and the gauge has a linear response.

[Rembrant]

Mine behaves the same way. First half of a full tank drops fast then the bottom half drops slow.

[Machspeed]

Yep, when my gauges were working that is exactly how they behaved. Quick from full then slows at mid tank. Truck bought brand new.

[myrl883]

Sounds like a theme here. My '86 only has one working fuel pump, so I only use the front tank, but it acts the same as the rest of you report. Full is full, and empty means empty, but it goes fast from full to half, then slows noticeably.

[Gary Lewis]

Even my 90's FDMs do the same thing.

[ArdWrknTrk]

And this is a nice feature of the TechnoVersions MeterMatch.
You can calibrate intermediate settings and have the needle pointing at a "true" position.

Since mine is around 150 miles to where I want it to read empty, I'm going to put 50 miles on a full tank, set it to 2/3, and go 100 miles and set it for 1/3.
(I haven't gotten this far yet, but it's coming)

[Rembrant]

ArdWrknTrk wrote

And this is a nice feature of the TechnoVersions MeterMatch.
You can calibrate intermediate settings and have the needle pointing at a "true" position.

What are you talking about here Willis? Fill me in.

[Gary Lewis]

Jim and I are both running TechnoVersions MeterMatch units.  They are supposedly able to match about any fuel level sender to any gauge, but we are using the later sending units with Bullnose gauges.  And that is quite a feat as the later senders not only have a different resistance range, but empty is full and vice versa.

[ArdWrknTrk]

Gary and I have a calibration unit wired between senders and gauge.

He has '90's FDM's in a Bullnose cluster and I have Bullnose senders and a Bricknose dash.

Bullnose are 10-70 ohms and Brick are 16-160ish.
But they read in opposite directions.

Gary had plans to use Arduino to fix the problem, but this little 1/2 x 2 x 3" box does it all and can have intermediate points made if the sender is not a linear signal.

I think mine was $65?

[Gary Lewis]

Oh yes, I forgot you have Bullnose senders and Bricknose gauges.  

[ArdWrknTrk]

My '87 is a "one year wonder" with mechanical pump and 16-160 ohm senders.

I can (did) get large bung '85-86 senders.
But my truck would below empty when full and about 7/16 scale when it ran out.

I wanted to point out the whole "drops like a rock, then runs forever" situation can be tuned out by the MeterMatch.

[rcarlisle]

Found this thread in a search.   Just wondering about fuel levels.   I've filled up twice now.  Each time the needle was on E or close. Didn't want to run out so filled it.  First time it took 8 gals and second was 11.   Second time was a bit lower.  

My mom had a 78 Granada with similar instruments.   The fuel gauge basically had 6 quarter tanks.   A quarter over F, a quarter under E.   Wondering if the 81 is going to be the same?  PO has reciepts for new tank and sending unit, but no idea what they are.  

Do I just adapt?  Is there something to do?  to check?  adjust?   Easy?  I'm not taking bed off - I'll just fill it up and keep experimenting with how low I can go without running out, if it's too complicated - everything works, just not like I would prefer.  It does seem consistent at least.  And the mileage is bad right now (10-ish), but there has been quite a bit of idling, and just running around the block in the first couple weeks.  And as Rat Dude said, it drops quick then seems to level out on the gauge.  

[Gary Lewis]

Before I had two tanks working I drove around with a 5 gallon can of gas in the back until I had a good understanding of how far it would go.  And it seems that every fuel sender is very different.  But many go for a long time on empty.

[1986F150Six]

Once one determines their truck's actual gas mileage, the odometer becomes a good friend!